Ground start type two-way trunk circuits between a central office and customer terminal equipment as presently designed by many manufacturers are subject to a condition referred to in the telephone industry as "glare." Glare is defined as a condition wherein the two wire trunk circuit can be seized from both ends simultaneously or, after release by the central office, the customer terminal equipment trunk circuit can again be seized by a new call from the central office prior to readying of the customer terminal circuits to a proper condition to receive such call.
As an example, the customer terminal equipment might be a ground start private automatic branch exchange (PABX) subject to receiving several telephone calls put through a central office. After release or termination of the call from the central office and after the forced release of the two wire trunk at the PABX terminal by a customer at a station serviced by the PABX, there is an interval of time of approximately three hundred or more milliseconds during which the PABX trunk circuits can be seized by a new call prior to release of the customer station on the previous call, even though the customer has hung up his telephone. When this situation occurs or when simultaneous attempts to seize the trunk wire line from both ends occur, the referred to condition of "glare" exists.
It would be highly desirable if the foregoing "un-guarded" interval of time could be substantially reduced thereby minimizing the possibility of "glare." The proper readying of the PABX trunk circuits to receive a subsequent call after release of a previous call depends upon the release and operate time of line equipment relays and the like at the central office. Since these latter circuits require a finite time to operate, it is not possible to reduce the referred to un-guarded time interval to zero. Thus, there is virtually no way to guard against a simultaneous seizure from each end. On the other hand, the operating time of the line equipment relays and circuits of the central office is sufficiently small that the unguarded interval involved could be substantially reduced.